Lab: xv6 lazy page allocation

One of the many neat tricks an O/S can play with page table hardware is lazy allocation of user-space heap memory. Xv6 applications ask the kernel for heap memory using the sbrk() system call. In the kernel we've given you, sbrk() allocates physical memory and maps it into the process's virtual address space. However, there are programs that use sbrk() to ask for large amounts of memory but never use most of it, for example to implement large sparse arrays. To optimize for this case, sophisticated kernels allocate user memory lazily. That is, sbrk() doesn't allocate physical memory, but just remembers which addresses are allocated. When the process first tries to use any given page of memory, the CPU generates a page fault, which the kernel handles by allocating physical memory, zeroing it, and mapping it. You'll add this lazy allocation feature to xv6 in this lab.

Before writing code, you should make sure you have read "Chapter 3: Page Tables" from the xv6 book and studied the corresponding code.

In this and future labs you will progressively build up your kernel, but it will be handy to maintain a branch per lab: your previous solutions: XXX actually not progressive; fix this...

  $ git fetch
  $ git checkout lazy
XXX make the alloc branch in xv6-riscv-fall19

Eliminate allocation from sbrk()

Your first task is to delete page allocation from the sbrk(n) system call implementation, which is the function sys_sbrk() in sysproc.c. The sbrk(n) system call grows the process's memory size by n bytes, and then returns the start of the newly allocated region (i.e., the old size). Your new sbrk(n) should just increment the process's size (myproc()->sz) by n and return the old size. It should not allocate memory -- so you should delete the call to growproc() (but you still need to increase the process's size!).

Try to guess what the result of this modification will be: what will break?

Make this modification, boot xv6, and type echo hi to the shell. You should see something like this:

init: starting sh
$ echo hiusertrap(): unexpected scause 0x000000000000000f pid=3
            sepc=0x0000000000001258 stval=0x0000000000004008
va=0x0000000000004000 pte=0x0000000000000000
panic: uvmunmap: not mapped
The "usertrap(): ..." message is from the user trap handler in trap.c; it has caught an exception that it does not know how to handle. Make sure you understand why this page fault occurs. The "stval=0x0..04008" indicates that the virtual address that caused the page fault is 0x4008.

Lazy allocation

Modify the code in trap.c to respond to a page fault from user space by mapping a newly-allocated page of physical memory at the faulting address, and then returning back to user space to let the process continue executing. You should add your code just before the printf call that produced the "usertrap(): ..." message. Your solution is acceptable if it passes usertests.

A good way to start this lab is by fixing usertrap() in trap.c so that you can run "echo hi" in the shell again. Once that works, you will find some additional problems that have to be solved to make usertests to work correctly. Here are some hints.

If all goes well, your lazy allocation code should result in echo hi working. You should get at least one page fault (and thus lazy allocation) in the shell, and perhaps two.


Now you have the basics working, fix your code so that all of usertests passes:

Your solution is acceptable if your kernel passes usertests:

$ usertests

This completes the lab. Commit your changes and type make handin in the lab directory to hand in your lab.